This invention relates to sheet and sheet-metal products, including exterior automotive body components, such as fenders or the like, made of non-heat-treatable aluminum alloys, particularly alloys in the 5000 Series (Aluminum Association designation) which consist essentially of aluminum and about 2-8% magnesium by weight.
It is well recognized that magnesium-containing aluminum alloys in the 5000 Series have good formability in an annealed condition but their use in the manufacture of sheet-metal products has been limited due to objectionable strain-induced marks or imperfections which develop on the surface of the sheet during forming. These marks, otherwise known as "Luder lines" or "stretcher-strains", can be so pronounced as to be visible even when the surface of a sheet-metal product is painted.
Generally, there are two varieties of Luder lines which may be of concern. These are sometimes respectively designated as "Type A" and "Type B" lines. Type A lines refer to random, wedge-shaped markings developed at low levels of strain, often as little as about 1/4 percent, while Type B lines refer to parallel ripples that occur at somewhat higher levels of strain such as about 2 percent.
At levels of strain of about 1.5 to 2.5 percent, Type A lines tend to disappear. Under laboratory conditions, when sample strips are slowly stretched in a tensile test machine, disappearance of Type A lines typically coincides with the onset of Type B lines.
In actual practice Type B lines rarely present a problem to the manufacturer. The formation of Type B lines is suppressed, for example, where the forming of the sheet is carried out with sufficient speed. Type B lines may also be avoided at relatively slow forming speeds under certain biaxial strain conditions.
Type A lines, however, are not so readily controlled as Type B lines and tensile test machine results are fairly indicative of what can be expected in practice.
Several ways to counteract the tendency of Al-Mg alloys sheet to display Type A Luder lines have been heretofore described in the literature, but it does not appear that any of these have been carried out commercially.
In accordance with one approach to the problem, the grain size in the metal is carefully controlled to a relatively coarse 0.04 to 0.06 mm by means of a substantial variation in conventional mill practices. In making sheet of 1/32 to 1/16 inch finished thickness, for example, the metal is conventionally hot rolled to a convenient reroll gauge such as about 0.10 to 0.25 inch in thickness, sometimes being followed by an intermediate annealing operation, after which it is cold rolled from the reroll gauge to the finished thickness. This cold rolling operation effects at least a 40 percent and often as much as 60 to 80 percent reduction in thickness, and consequently work hardens the material appreciably. To restore formability, the metal is given a recrystalizing anneal by heating within the range of 650.degree. to 950.degree. F.
In order to achieve the 0.04 to 0.06 mm grain sizes, however, it is necessary to perform the cold rolling operation in successive stages interrupted by still additional intermediate annealing operations. This of course complicates production practices and involves substantial added expense. A further disadvantage to the approach is that if grain size becomes as coarse as about 0.07 mm or more, an objectionable roughening of the metal surface, known as "orange peel", can result.
A second recognized method for dealing with the problem of Type A lines involves conducting the usual annealing step at a temperature of about 930.degree. F. (although some have reported good results at temperatures as low as 572.degree. F.), then quenching the metal in water, preferably cold water, and finally lightly rolling or roller-leveling so as to flatten the sheet without materially pre-straining it. Drawbacks to this method include the higher energy requirements and inflexibility of being forced to anneal at the higher end of the temperature scale. It is also significant to note that on being aged, metal treated by this method is reported to revert to its tendency to show Type A lines when formed.
According to another approach, the metal is prestrained by following a conventional anneal with: (1) a cold rolling pass to effect a thickness reduction of about 5 to 10 percent, (2) a heavy roller-leveling operation, or (3) a stretching of the sheet to effect a permanent set of about 11/2% or more. Nominal cold rolling has been proven to be impractical for execution on a commercial scale, however, since the precise degree of work put in is not readily susceptible to control and the properties of the metal can be expected to differ considerably with slight variations in sheet gauge. And with each of the three stated modes of pre-straining, a substantial loss of ductility has been a recognized disadvantage.
To obviate the loss of ductility accompanying the immediately preceding approach, in accordance with still another proposal, cold rolling or roller-leveling would be followed by a non-recrystalizing anneal. In carrying out such a procedure, however, experience has shown that the properties of the metal are unduly sensitive to time and temperature, and are thus most difficult to control. Furthermore, the need for an added annealing operation is an obvious drawback involving time and energy.